1. Field of the Invention
The invention relates to biocompatible carbonaceous films for applications including medical implantation and to a method for fabricating the films on a substrate surface.
2. Prior Art
Elemental carbon occurs naturally in two widely known allotropic forms: diamond and graphite, each of which exist in more than one polymorphic modification. Diamond is a 3-dimensional spatial polymer of tetrahedral carbon in which every carbon atom is bonded to four other carbon atoms by four identical bonds, each 1.54 xc3x85, long. Diamond, which is a dielectric, has a minimal structural unit consisting of a tetrahedron, with carbon atoms occupying positions in each of the tetrahedron""s corners and at the center of the tetrahedron.
Graphite consists of one or more 2-dimensional (planar) polymer sheets of trigonal carbon wherein the polymeric sheets form parallel layers. Each carbon atom is bonded to three other carbon atoms with three identical bonds evenly distributed in a plane, each bond being 1.42 xc3x85 long. The identical overlying graphite layers are oriented parallel to each other and are located at a distance of 3.35 xc3x85 from each other. Graphite is a conductor of electric current. The 6-carbon benzene ring is the basic structural unit of graphite.
Carbyne is the third known allotropic form of polymeric carbon. The structure of carbyne is the most similar to the structure of Tetracarbon(trademark), the polymeric form of carbon referred to hereinafter as Tetracarbon, which comprises the subject matter of the present invention and is defined. Carbyne is a semiconductor formed from linear polymeric carbon. A straight carbon chain is the basic structural element within a carbyne layer in which every carbon atom is bonded to two neighbors with two equal bonds, wherein each bond is between 1.19-1.38 xc3x85 long and the distance between carbon chains is 2.97 xc3x85. A minimal structural unit from which a carbyne crystal can be assembled is a hexagonal prism. Bent chains are located in the corners of the hexagon. Bendings divide the prism into two parts. A straight chain is located in the center of the lower part with a comparable chain being absent in the upper part. Admixture of hetero (non-carbon) atoms may result in such hetero atoms occupying this vacancy. Carbyne was obtained for the first time in 1969 by means of oxidizing polydehydrocondensation of acetylene. Carbyne forms a sheet-like microcystal consisting of a plurality of regularly shifted chemically bonded Axe2x80x94Bxe2x80x94Axe2x80x94B . . . layers. Each A layer comprising the microcrystal consists of densely packed carbon chains oriented perpendicular to the plane of the layer and sandwiched between two B layers. A and B layers are regularly shifted relative to each other and chemically bonded to adjacent layers. In each B layer there is a regular grating of chain vacancies. At present, no carbyne crystals are known having a size greater than 1 xcexcm (Bulletin of the Russian Academy of Science. Physics, 1993, vol. 3, p. 450).
In addition to the pure crystalline allotropic forms of carbon described above, there are a number of intermediate transitional forms such as pyrolytic carbon and glassy carbon. Pyrolytic carbon is a synthetic high-density carbon polymeric with turbostratic structure and composed of either pure or silicon-alloyed carbon microcrystals. These properties distinguish pyrolytic carbon from other polymeric carbon materials such as graphite, diamond and glassy carbon. Short range order in a pyrolytic carbon film which presents a turbostratic structure wherein the carbon chains are in a plane parallel to the plane of the film and is similar to that of graphite; the basic structural unit being 6-carbon slightly-deformed benzene rings. Pyrolysis of a gaseous hydrocarbon is employed for depositing pyrolytic carbon upon a substrate surface. The high temperature required for pyrolytic deposition limits the choice of substrate to materials to those which are stable at high temperatures such as ceramics and low-porosity graphite. In addition, a substrate composed of a brittle material such as graphite must first be mechanically shaped prior to coating. Due to the extreme hardness of pyrolytic carbon, it can only be worked and polished with diamond tools and pastes so that only relatively simple shapes are suitable for graphite substrates.
Vapor deposition has been used to transfer carbon atoms from a turbostratic carbon target to a substrate such as the surface of an implantable prosthesis. By appropriately regulating the conditions under which carbon deposition takes place, it is possible to hold the temperature of the substrate below a predetermined limit so as to minimize or prevent altering the substrate""s physical characteristics. Vapor deposition allows carbon to be deposited in a thin film upon a substrate surface, the film forming a coating which retains the turbostratic structure and high-density characteristic of pyrolytic carbon.
Representative patents and author""s certificates describing various prior art carbon coatings, including turbostratic coatings, are presented below in Table I.
A method for manufacturing a polymeric prosthesis having a biocompatible carbon coating is shown in U.S. Pat. No. 5,133,845. The biocompatible carbon coating is deposited on the substrate surface by means of triode cathode spraying. Carbon is sprayed at low temperature at a pressure ranging from 6xc3x9710xe2x88x924-6-10xe2x88x923 mbar (6xc3x9710xe2x88x922-6xc3x9710xe2x88x921 Pa). Spraying voltage is 2000-3200 V, the spraying current being between 0.1-0.3 Amperes. A uniform biocompatible coating of turbostratic carbon is formed upon the substrate surface with the density of the coating being at least 2.1 g/cm3.
Another method for manufacturing a prosthesis having a biocompatible film coating is presented in U.S. Pat. No. 5,084,151. The coating deposition proceeds in a vacuum chamber at a pressure of 10xe2x88x924-10xe2x88x922 mbar. A plasma beam is formed and directed toward a carbon cathode disposed to lie in the path of the plasma beam. High voltage at low current is applied to the cathode. The sprayed carbon atoms are directed toward and impinge upon the substrate surface which is heated to a temperature of 250xc2x0 C. The coating obtained by this method also has turbostratic structure.
A turbostratic carbon polymer film can be applied to the outer surface of a prosthesis in an apparatus comprising a power supply and a vacuum chamber partitioned to form two sub-chambers. A gaseous ion source directs an ion beam through an aperture in the first sub-chamber into the second sub-chamber. In the second sub-chamber, which is open to (in gaseous communication with) the first sub-chamber, a carbon cathode is located directly in the path of the ion beam. A ring-shaped anode surrounds the carbon cathode. A heat transfer system is employed for cooling the carbon cathode and anode. The carbon cathode is sprayed with the ion beam and carbon is vaporized. The substrates to receive the coating are placed within the second sub-chamber and disposed to receive the carbon vapor on the surface thereof upon carbon vapor condensation. This method and apparatus produces a turbostratic carbon film which is deposited upon a substrate surface to form a coating on the substrate which is reported to exhibit biocompatible properties.
Carbyne coating has been reported to posses high biocompatibility and thromboresistivity (Diamond and Related Materials, v.4 (1995) p.1142-44). Carbyne coatings, fibers and films are prepared by the chemical dehydrohalogenation of halogen-containing polymers such as, for example, polyvinylidene fluoride (xe2x80x9cPVDFxe2x80x9d). An alkaline alcoholic solution is used as the dehydrohalogenating agent. However, such carbyne coatings can be produced only on the surface of PVDF substrates which limits its applications.
A method for effecting the ion-stimulated deposition of carbyne on a substrate surface is known (Bulletin of the Section of Physics of the Academy of Natural Sciences of Russia, no. 1, 1993, p. 12). The method relies on the ion-stimulated condensation of carbon in high vacuum (10xe2x88x927 Torr). A flowstream of carbon and a flowstream of ions of inert gas (e.g. argon), either simultaneously or sequentially, are directed to impinge upon the substrate surface. The carbon flux is obtained by means of thermal evaporation or ion spraying of graphite. The energy of the argon ions (Ar+) bombarding the substrate surface may vary, but for deposition is generally within the energy interval between 90 up to 200 eV. The current density of ions at the substrate is 10-1000 xcexcA/cm2, the rate of film growth is 10-1000 xc3x85/min, and the thickness of the deposited film is 200-1000 xc3x85. Carbyne films are obtained by means of irradiation with ions either simultaneously or alternating with condensation of carbon. The resulting films are quasimorphous, consisting of an amorphous carbon matrix and microcrystalline impurities. The method is inoperable for coating surfaces having either a relatively large area and/or a complex shape, and may be applied only for the deposition of films on conducting or semiconducting substrate surfaces. The method is inoperable for depositing carbyne on the surface of substrates such as ceramics, non-conducting polymers and silicone rubber which are substrate materials commonly used for manufacturing medical implants.
In summary, the prior art does not provide either an apparatus or a method for depositing a non-turbostratic carbon film having a structure as described below or an apparatus operable for depositing a non-turbostratic carbon film on a large surface, wherein the film exhibits the properties characterizing Tetracarbon which are more fully disclosed below.
It is an object of the invention to provide a biocompatible coating for a surgically implantable article.
It is a further object of this invention to provide a non-turbostratic carbon film adapted for coating a substrate surface.
It is a further object of the invention to provide a method for making a prosthesis or similar surgically implantable device which has a biocompatible tissue-contacting coating on the outer surface.
It is yet a further feature of the invention to provide an apparatus which is operable for depositing a non-turbostratic biocompatible polymeric coating upon the surface of a substrate.
A further objective of the invention is to provide a coating for a surgically implantable medical device wherein the coating is adapted to permit self-reassembly in order to accommodate tissue ingrowth.
Tetracarbon is a polymeric carbon film having a non-turbostratic 2-dimensional planar structure. In Tetracarbon films the short, straight linear carbon chains that form the layer are organized into densely packed hexagonal structures with the distance between chains being 4.8-5.03 xc3x85. Unlike turbostratic carbon films, in Tetracarbon film the long axis of the linear carbon chains comprising the film ares oriented perpendicular to the plane of the film. A Tetracarbon film may be a single layer or many layers which overlie one another. If the number of layers in a Tetracarbon film exceeds one, the layers are identical and randomly shifted relative to each other. In Tetracarbon, the interaction between the linear carbon chains in the film is due to van der Waals forces which set the distance between the chains in the range 4.8-5.03 xc3x85. As is true with carbyne, a carbon chain is the main structural element of Tetracarbon. The Tetracarbon chain consists substantially entirely of carbon atoms, each carbon atom having two 1.19-1.38 xc3x85-long valence bonds with a 180xc2x0 angle between them. The introduction of hetero atoms into a carbon chain under the influence of ion irradiation and alloying can modify the structure of Tetracarbon be to adapted to particular applications. The morphological features characterizing a Tetracarbon coating can be modified, for example by:
(a)xe2x80x94regular joining of chains within adjacent layers;
(b)xe2x80x94splitting of a chain into linear fragments; and/or
(c)xe2x80x94formation of bends within a carbon chain; and/or
(d)xe2x80x94changing the distance between carbon chains.
The length of linear carbon chain fragments and the number of bends effect the morphology of Tetracarbon. Thus, the morphology may be varied by the choice of gas used for ion irradiation, the composition using an admixture of gases and varying the proportions of the admixture and the temperature of deposition. Tetracarbon structure may xe2x80x9cself-organizedxe2x80x9d in vivo; structurally readjusting to adapt itself to the structure of a protein molecule growing on and intimately into the Tetracarbon due to the interaction between the film and the protein penetration of endogenous ions into the Tetracarbon layer.
The above objectives are met with a polymeric carbon film referred to herein as Tetracarbon. Tetracarbon refers to a carbonaceous polymeric film, the surface of the film defining a plane. The film may be either a single layer or a superimposition of multiple layers wherein each layer within the film consists essentially of a plurality of linear chains of covalently bonded carbon atoms. The linear (end to end) axis of each linear carbon chain in a layer is perpendicular to the plane of the film surface. Thus, Tetracarbon is a non-turbostratic material. Only one end of the carbon chains comprising the innermost layer of Tetracarbon may be bonded to the surface of the substrate upon which the Tetracarbon layer is deposited. The opposing end of the carbon chains project away from the substrate surface in a substantially vertical direction.
An apparatus operable for depositing a Tetracarbon coating upon a substrate surface comprises essentially a vacuum chamber inside which are disposed in combination: a graphite cathode of main discharge, an anode of main discharge; an ignition electrode, a cathode of auxiliary discharge separated from the ignition electrode by a dielectric spacer; and a power supply. The vacuum chamber has two side compartments, each of which are in gaseous communication with the interior of the vacuum chamber by means of apertures therebetween. One of the two side compartments contains the cylindrical graphite cathode of main discharge and the anode of auxiliary discharge, surrounding the cathode of main discharge with a gap therebetween. The end of the cylindrical anode of auxiliary discharge closest to the substrate has a conic shear directed axially inward and facing the cathode of main discharge. The anode of the main discharge comprises two or more electrically conductive parallel rings which are rigidly connected to one another by metal rods. The ignition electrode, dielectric spacer, and the cathode of the auxiliary discharge are fabricated as a laminated ring, each of the elements being rigidly affixed to each other and interposed between the anodes of the main and auxiliary discharges. The anode of auxiliary discharge, cathode of main discharge, ignition electrode, cathode of auxiliary discharge, dielectric spacer and anode of main discharge are coaxially disposed with respect to each other.
A substrate holder, placed inside the vacuum chamber behind the anode is adapted to support a substrate and permit planetary rotation of the substrate around two axes and is connected electrically to the chassis ground of the vacuum chamber. The axis around which the substrate holder revolves is tilted or inclined with respect to the orbital axis. An aperture in the wall of the second side compartment of the vacuum chamber permits entry of an ion beam into the vacuum chamber. The ion and plasma beams intersect at the substrate surface. The apparatus also includes a capacitor and an inductance, one pole of the inductance being connected to the cathode of main discharge and the other pole being connected to a negatively charged plate of the capacitor, the positively charged plate of which is connected to the anode of main discharge. The poles of the power supply are attached to the corresponding plates of the capacitor. The cathode of main discharge is made of graphite having high purity. For medical applications, a purity of 99.99% or better is preferred.
While the above summary of the invention generally sets forth the nature of the invention, the features of the invention believed to be novel are set forth with particularity in the appended claims. However, particular embodiments of the invention, both as to organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: